I had the fortune, early in my career, to be mentored by a meticulous German engineer named Horst Pimmler. Mr. Pimmler was the founder of Monticello Spring Corporation and had a way to classify everything. His way of classifying compression springs made it possible to group just about any compression spring into one of five categories, based on their requirements.
This method was predicated on the notion that certain callouts defined a definite tolerancing scheme. This made it easy to relate to what should be toleranced and what should be referenced. Since spring engineering is rooted in time-tested physics and mathematics, fitting a given design into a “type” was a natural flow. Any parameter requiring tolerancing would follow the SMI industry guidelines, while the referenced parameters were allowed to vary as needed to meet the critical toleranced requirements.
TYPE ONE: This is a dimensional only design. This means there are no load or rate requirements. This is the simplest of types and only requires conformance at the dimensional level—wire diameter, OD, coils, and free length. All the parameters are toleranced, except the number of coils, which is locked in with no variation required. This means any load or rate will vary somewhat with the inherent variation of material and coiling, but inspection is simplified and any load requirements are not critical to function.
TYPE TWO: This is a design with only a spring rate requirement—the rate is toleranced. Spring rate is largely a function of active material—this is the most expedient parameter to adjust for conformance. This type allows a tolerance on every dimensional parameter except the coils. Coils are allowed to vary to adjust rate to nominal.
TYPE THREE: This is a design with only a single load requirement at a height—a single load is toleranced. Single loads are largely a function of free length—this parameter is easily adjusted and can obtain a nominal load by tweaking the free length. This type allows a tolerance on every dimensional parameter except free length. Free length is allowed to vary to adjust load to nominal. Coils are locked in.
TYPE FOUR: This design has both a single load at a height and a rate requirement—a single load is toleranced and the rate is toleranced. This type is a combination of both Type Two and Type Three. Therefore, both coils and free length should be referenced (or generously toleranced) to allow adjustment to nominal of both load and rate. This type allows a tolerance on the wire size and OD, but not the coils or free length. Free length is allowed to vary to adjust load to nominal, and coils are allowed to vary to adjust rate to nominal.
TYPE FIVE: This design has two load requirements at two designated heights—two loads are toleranced. A two load requirement demands a given rate so this essentially requires the same tolerancing as a Type Four. This type allows a tolerance on the wire size and OD, but not the coils or free length. A two load spring design does, however, require a sequence of events in the proper order. The rate must first be obtained because the two loads depend on the proper rate. Then, the free length can be adjusted shorter or longer as needed to bring both loads to nominal. The rate must always be adjusted first, then the free height since the two loads cannot be realized unless the rate is correct.
Dimensional only—tolerance all parameters, lock in the number of coils
Rate only—tolerance all parameters except coils
Load only—tolerance all parameters except free length
Load and Rate—tolerance all parameters except coils and free length
Two loads—tolerance all parameters except coils and free length
One last comment: these are guidelines, not laws. The customer still defines what is critical to its needs. But this is the science of it all and if these guidelines can be followed, the requirements on all five types are more easily obtained, which reduces set up times, production time and reduces overall shop hours.
And hats off to Mr. Pimmler.
By: Randy DeFord, Engineering Manager Mid-West Spring & Stamping